1. Field of the Invention:
The present invention relates to the field of color television and in particular to the control and adjustment of the separate guns of a three gun cathode ray tube to preserve "gray scale." Gray scale is preserved when a normally hueless object remains hueless from low to high states of white illumination.
2. Description of the Prior Art:
At the factory, after the circuit of a television receiver has been assembled, it is customary to "set up" the color of the receiver. Assuming that it has a three gun tube, the television receiver requires the color set up process so that the reproduction of the separate color components will be standardized. The color set up process is designed to adjust each gun so that the emission of each will "track" through variations in picture brightness, and produce a hueless rendering of hueless subject matter. The main independent variable of gun emission are well recognized: -- the gain of each gun and the gun cut-off voltage. It is a common practice to provide adjustments for each of the six (five) parameters (i.e., two per gun), perhaps omitting one gain adjustment. The adjustments require several potentiometers, which are expensive components, and thus add the costs of the television receiver. In addition to part cost, a second disadvantage is in the complexity of the factory color set-up process requiring considerable operator time. The operator finds that the adjustments interact, and an iterative adjustment procedure is conventionally required to insure propr gray scale throughout the brightness range. A third disadvantage is that the color set-up process is not permanent. If the cut-off voltage changes, or the gains of the guns change which occurs as the CRT ages, -- or the external circuit changes, the gray scale may need readjusting.
One known approach has been to sense the cathode beam currents and apply a corrective d.c. bias to the grids along with the video drive. This requires six accessible terminals on the three gun cathode ray tube. The present approach seeks to perform both the sensing and video drive function on the cathode terminals. Better performance and economy are sought by the present approach. The present approach is suitable for a cathode ray tube design in which single grid and single screen grid structures are used for the three guns.